Megakaryocytes (MKs) are responsible for the production of platelets, which have a major impact on blood homeostasis. MK is unique among blood cells in attaining states of polyploidy, which precedes platelet biogenesis. Uncontrolled MK proliferation and deregulated polyploidization are hallmarks of some blood cell pathologies. Our recent studies implicate Lysyl Oxidase (LOX) in the regulation of MK ploidy and proliferation. This revised competitive renewal application investigates the role of LOX in these processes. LOX is a known extracellular regulator of the fibrogenic response, acting by cross-linking matrix proteins. Although the role growth factors released by MK, such as Platelet-Derived Growth Factor-ss (PDGF-ss), has been documented in the expansion of the MK lineage in MK leukemia-associated myelofibrosis, many aspects of this condition remain elusive, and current treatments are largely ineffective. Our preliminary findings indicate that LOX is expressed at low levels in a pool of normal MKs, primarily in the low-ploidy cells. However, LOX is abundant in displastic MKs associated with myelofibrosis, and is upregulated by PDGF-ss. We found that inhibition of LOX enzyme activity significantly diminished PDGF-ss binding to cells and PDGF-ss-induced expansion of MKs. We also made the novel observation that LOX-PP, a product of pre- LOX processing, inhibits MK polyploidization. Based on these findings, we hypothesize that upregulated LOX in displastic or leukemic MKs has a pivotal role in MK expansion, inhibition of polyploidy, and matrix deposition. To test this, we propose three specific aims: 1. Elucidate the mechanism of effect of LOX-PP on MK polyploidization; 2. Investigate the mechanism of effect of LOX on PDGF-ss-induced MK expansion; and 3. Determine the consequences of altered LOX expression in vivo on MK expansion, polyploidy and myelofibrosis. Understanding how LOX and LOX-PP regulates MK proliferation and polyploidization will identify new targets for control of these processes, informing the development of new therapeutic interventions for MK-related disorders and providing further insight into the physiological control of MK ploidy and proliferation.